The 'D' type retroviruses comprise a group of primate reverse transcriptase-containing viruses that differ from the 'B' and 'C' type oncoviruses in both morphogenesis and biological properties. One member of the group, the Mason-Pfizer Monkey virus (M-PMV) has been implicated in both non-human, primate and human mammary carcinogenesis. It is proposed to isolate and characterize both conditional (temperature-sensitive) and non-conditional replication mutants of M-PMV. These mutants will be used to investigate the role of viral genes in virus replication and cell fusion by M-PMV. Conditional and non-conditional mutants will be used to assign positions of viral genes on the genome by oligonucleotide mapping experiments. The possibility that M-PMV carries a gene for cellular transformation will be investigated. The potential for genetic recombination between 'D' type viruses will be studied.